1. Field of the Invention
The present invention relates to a capacitive ultrasound transducer configured by having an electret and an electronic device.
2. Description of the Related Art
Piezoelectric elements made of a ceramic piezoelectric material PZT (lead zirconate titanate) have been chiefly used as an ultrasound transducer. In recent years, a capacitive ultrasound transducer such as the one disclosed in Japanese Patent Application Laid-Open Publication No. 2005-510264 has attracted attention.
The capacitive ultrasound transducer is configured by having a pair of electrodes formed of an upper electrode and a lower electrode facing each other through a gap portion formed therebetween, and transmits or receives ultrasound through vibration of a membranous portion including the upper electrode (also referred to as “membrane” or “diaphragm”).
The capacitive ultrasound transducer converts an ultrasound signal into an electrical signal on the basis of changes in electrostatic capacity between the upper and lower electrodes when receiving ultrasound and, therefore, requires application of a DC bias voltage between the upper and lower electrodes particularly at the time of reception.
From the viewpoint of reducing the power consumption and size of an ultrasound transducer, it is preferable to reduce or set to zero the voltage value of the DC bias voltage. As a technique to reduce the DC bias voltage, a technique of producing a potential difference between the upper and lower electrodes of the capacitive ultrasound transducer by providing between the upper and lower electrodes an electret film holding electric charge is known.